But one role that Mr. de Blasio quietly and fully inhabits every day is that of small-time landlord, at a modest duplex one block west of his home in Park Slope, Brooklyn.

Covered in light gray siding and fronted by a thin strip of porch and a patch of concrete, the duplex was bought for $612,500 in 2004 by Mr. de Blasio’s mother, Maria Wilhelm. Then living alone in Westchester County, she wanted to be closer to her son and his family, and Mr. de Blasio and his wife, Chirlane McCray, were co-signers of the mortgage. The duplex, on 11th Street, is 500 feet away from the townhouse where the couple live with their children, Chiara and Dante.

After his mother died in 2007 at age 89, Mr. de Blasio, then a city councilman, rented out both apartments: a two-bedroom on the ground floor and a one-bedroom on top.

“I don’t have one negative word to say about Bill and Chirlane,” said Ellen Mittelholzer, 30, a French teacher who lives in the second-floor one-bedroom apartment with her husband, a software engineer. After Ms. Mittelholzer and her husband found the apartment on Craigslist, the real estate agent tipped them off that the owner was the public advocate. “Great; I don’t know what that is,” Ms. Mittelholzer said she thought to herself.

The real estate agent also said that there was a vetting process and that the owners wanted to meet the prospective tenants. So an hour before signing the lease, the couple went to Mr. de Blasio’s home, where they were warmly greeted by Mr. de Blasio and Ms. McCray, ushered into the living room and offered tea and cookies. “We chatted for an hour, and they’re completely down to earth,” Ms. Mittelholzer said.

Mr. de Blasio also swiftly responded whenever tenants reported problems, which they say are rare. Less than 24 hours after tenants called with news that the water was not hot enough, a repairman was sent to clean the heater. The response was also quick for the broken dryer. Come winter, Mr. de Blasio would stop by to shovel snow and sprinkle salt over ice, or would send his son, Dante, in his stead.

“They personally make sure things are taken care of and taken care of quickly,” said Jennifer Way, 38, a children’s book editor who lives in the first-floor apartment with her husband, Daniel Wilmer, 42, and their 6-month-old daughter.

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Stephanie Wolf, a publicist who lived in the ground-floor apartment with a boyfriend for a couple of years, until about 2009, said the advantages of being a de Blasio-McCray tenant extended long after the lease was up. They have furnished her with referrals for new apartments and with reference letters. After Hurricane Sandy hit, she volunteered at a church in Far Rockaway, Queens, that was dark and frigid because it had lost power, so she called Mr. de Blasio on his cellphone to tell him. “He picked up right away and made a bunch of phone calls and put his team on a project to get generators to the church,” Ms. Wolf said. “Living in New York, you have good landlords and bad landlords. As landlords go, Bill’s at the top.”

The women would not disclose what they paid in rent — one-bedrooms in the neighborhood typically rent for about $2,500 — but all said they were fair market prices.

The value of the duplex has soared to $1.1 million, according to the city’s Finance Department. Last week, Crain’s New York Business reported that Mr. de Blasio failed to disclose the rental income to the Conflicts of Interest Board. His campaign has responded that because the expenses on the property — including property taxes, mortgage interest and depreciation — exceeded the revenue from rent, there was no net income to report. In 2011, for example, his tax return showed he earned $47,500 in rental income and had $62,200 in deductions. (His Republican opponent, Joseph J. Lhota, does not own rental property in the city.)

It is unclear who will oversee the little building once Mr. de Blasio becomes, as all forecasts suggest, the next mayor. He has demurred when asked whether he would leave Brooklyn for the official mayoral residence, Gracie Mansion, at East End Avenue and 88th Street in Manhattan, saying he was waiting until the election to decide. He has noted, though, that the mansion, a sprawling, Federal-style, wood-frame home with five bedrooms and two floors, was far roomier than his townhouse, which he could conceivably rent out to increase his income.

Yet the de Blasios have repeatedly declared their love of Brooklyn. Last month, in an interview with The New York Times, Ms. McCray said that the prospect of moving was a conversation the family had yet to have. “We haven’t gotten there yet,” she said, “but we will have to discuss it.”

Ms. McCray did say that Dante would remain a student at Brooklyn Technical High School should the family decamp to Manhattan — and she suggested that her husband might continue to help with transportation.

“If that happens, he will either take the train, which is good,” Ms. McCray said, referring to her son, “or Bill will drive him, same as we do here.”

A version of this article appears in print on October 29, 2013, on Page A25 of the New York edition with the headline: As Landlord of Brooklyn Duplex, de Blasio Gets High Approval Ratings. Order Reprints|Today's Paper|Subscribe